How to Insulate the Underside of a Floor

Measurements

• You'll need a tape measure to calculate how much insulation you'll need underneath the home. Measure the distance between floor joists so you will know what width insulation to buy, and then measure the length of the floor underneath the house. To make the calculation simple, count the number of bays in which insulation goes and multiply that by the length of one bay. This gives you the total amount of insulation you'll need for the floor in feet. Buy insulation that equals the dimension between joists. You can opt for a denser insulation with higher R-factors if your floor joists have depths greater than 4 inches.

Gear

• Wear a long-sleeved shirt, goggles, gloves and a face mask when you install insulation. Even though insulation isn't toxic, if it gets on your skin or in your eyes it's uncomfortable and itchy. Large shears help cut insulation to fit small spaces or around plumbing or electrical wires. If you buy faced insulation, staple the facing onto the outer edges of the joists or fold these tabs up when you install it. Fiberglass insulation is light enough to stay in place between joists when sized to the joist dimension correctly, but when you cover the insulation with a vapor barrier, it ensures the insulation will stay in place while preventing air from moving through it.

Installation

• Start at one end of a bay and install the insulation for the entire length of that bay. When placing the insulation between the joists, don't crush it; instead, lightly place it into the space, starting at the bottom and working up until the batt is in place. If you crush the insulation, you affect its capability to effectively resist cold or heat movement through your house. It's not the fiberglass in the insulation that makes it thermally resistant; it's the air space between the fibers that gives it the insulating qualities.

Vapor Barrier

• While insulation does offer resistance to thermal movement, it doesn't offer resistance to moisture or air movement. Add a vapor barrier or diffuser to the joists underneath the home after you install the insulation. Secure the barrier to the joists with a staple gun or hammer stapler. Overlap edges where you have to cut the barrier to fit. A line of silicone caulk or similar product adds a seal to the vapor barrier to prevent air from seeping between the vapor barrier joints.

Cement Slabs

• In homes with cement slabs, you can't insulate underneath the floor. But if you live in a cold climate, you can add extra thermal protection over the slab before adding the floor covering. Floating floors use an underlayment, which also acts as insulation; a carpet pad does double-duty as insulation on a slab covered with carpet.